The Controller service provides the API for orchestrating a collection of
debugger agents to perform debugging tasks. These agents are each attached
to a process of an application which may include one or more replicas.

The debugger agents register with the Controller to identify the application
being debugged, the Debuggee. All agents that register with the same data,
represent the same Debuggee, and are assigned the same debuggee_id.

The debugger agents call the Controller to retrieve the list of active
Breakpoints. Agents with the same debuggee_id get the same breakpoints
list. An agent that can fulfill the breakpoint request updates the
Controller with the breakpoint result. The controller selects the first
result received and discards the rest of the results.
Agents that poll again for active breakpoints will no longer have
the completed breakpoint in the list and should remove that breakpoint from
their attached process.

The Controller service does not provide a way to retrieve the results of
a completed breakpoint. This functionality is available using the Debugger
service.

ListActiveBreakpoints returns the list of all active breakpoints for the debuggee.

The breakpoint specification (location, condition, and expressions
fields) is semantically immutable, although the field values may
change. For example, an agent may update the location line number
to reflect the actual line where the breakpoint was set, but this
doesn't change the breakpoint semantics.

This means that an agent does not need to check if a breakpoint has changed
when it encounters the same breakpoint on a successive call.
Moreover, an agent should remember the breakpoints that are completed
until the controller removes them from the active list to avoid
setting those breakpoints again.

All agents attached to the same application must call this method with
exactly the same request content to get back the same stable debuggee_id.
Agents should call this method again whenever google.rpc.Code.NOT_FOUND
is returned from any controller method.

This protocol allows the controller service to disable debuggees, recover
from data loss, or change the debuggee_id format. Agents must handle
debuggee_id value changing upon re-registration.

UpdateActiveBreakpoint updates the breakpoint state or mutable fields.
The entire Breakpoint message must be sent back to the controller service.

Updates to active breakpoint fields are only allowed if the new value
does not change the breakpoint specification. Updates to the location,
condition and expressions fields should not alter the breakpoint
semantics. These may only make changes such as canonicalizing a value
or snapping the location to the correct line of code.

The Debugger service provides the API that allows users to collect run-time
information from a running application, without stopping or slowing it down
and without modifying its state. An application may include one or
more replicated processes performing the same work.

A debugged application is represented using the Debuggee concept. The
Debugger service provides a way to query for available debuggees, but does
not provide a way to create one. A debuggee is created using the Controller
service, usually by running a debugger agent with the application.

The Debugger service enables the client to set one or more Breakpoints on a
Debuggee and collect the results of the set Breakpoints.